opal_config_subdir.m4: do not export vars before invoking subdir configure

== Short version

Do not export special variables into the environment (e.g., LIBS,
LDFLAGS, etc.) when invoking subdir configure scripts.  This prevents
problems described in open-mpi/ompi#471.

== More detail

Exporing special env variables before invoking a subdir configure
script causes problems in some cases.  E.g., in open-mpi/ompi#471,
when the user configures with `--with-hwloc=/path/to/hwloc`, and that
directory is *not* in a default linker search location will cause the
libevent subdir configuration to fail.

This happens because:

1. We'll pass LIBS="-L/path/to/hwloc/lib -lhwloc" to the libevent
   configure script
1. Meaning: configure-generated executables will link successfully
1. But unless LD_LIBRARY_PATH (or some other
   tell-the-linker-where-to-find-things mechanism) includes
   /path/to/hwloc/lib, the executable can't run.

Specifically, the libevent "hey, does the compiler generate proper
executables?" check will fail, and configure will abort (because OMPI
needs libevent).

I checked the history: exporting these vars dates all the way back to
LAM/MPI.  I can't think of a reason why we need to export these
variables -- AC_CONFIG_SUBDIRs doesn't do it; subdir configure scripts
should be orthogonal from the upper-layer configure script (and its
variables).  So let's remove these export statements and see if
anything breaks.

config/opal_config_subdir.m4

@@ -121,11 +121,6 @@ if test "$subdir_dir" != ":" && test -d $srcdir/$subdir_dir; then
     # Invoke the configure script in the subdirectory
     #
 
-    export CFLAGS CPPFLAGS
-    export CXXFLAGS CXXCPPFLAGS
-    export FCFLAGS
-    export LDFLAGS LIBS
-
     sub_configure="$SHELL '$subdir_srcdir/configure'"
     AC_MSG_NOTICE([running $sub_configure $subdir_args --cache-file=$subdir_cache_file --srcdir=$subdir_srcdir --disable-option-checking])
     eval "$sub_configure $subdir_args \